This invention relates to methods and arrangements for high voltage single-stage variable speed drives for electric motors.
The Opal et al. U.S. Pat. No. 5,683,263 discloses a low and medium voltage pulse width modulated AC/DC power conversion method and apparatus by which AC power is converted to DC power through a transformer with multiple secondary power cells which may be shifted in phase and the maximum output of each power cell may be less than the line-to-line voltage. The power output can have variable voltage and current which can be controlled using pulse width modulation techniques.
The Hammond U.S. Pat. No. 5,625,545 discloses an electric drive arrangement for controlling medium voltage alternating current motors which includes a multi-phase power transformer with multiple secondary windings providing multiple phase power to multiple power cells each of which has a single-phase output and is controlled by a pulse width modulation controller. Primary and secondary windings in the power transformer may be star or mesh connected and the secondary windings may be shifted in phase. Since the power cells are connected in series, the maximum output voltage for each cell may be less than the maximum line-to-line voltage. Pulse width modulation may be effected using a bridge converter composed of semiconductor switches in the form of power transistors connected in an H-bridge configuration.
The Watanabe U.S. Pat. No. 5,657,217 discloses a multiple-coupled power converter comprising two converter circuits having switches which are driven by corresponding pulse width modulation generators, depending on the voltage and phase of an input AC waveform. Three-phase outputs are provided to a load through inductive reactors. The patent to Eckersley U.S. Pat. No. 5,402,054 describes a variable speed drive control for a three-phase AC motor utilizing a plurality of single-phase semiconductor power circuits. Within each of the power circuits SCRs are selectively switched to conduct upper and lower half cycles to the output waveform for a given phase. Multiple rectifier H-bridge circuits for each phase of a power system are disclosed in the Debruyne et al. U.S. Pat. No. 5,687,071, while the patent to Schantz, Jr. et al. U.S. Pat. No. 5,517,063 discloses plural Y-connected inverter circuits and the Yamaguchi U.S. Pat. No. 5,373,436 is generally directed to a multi-phase variable speed motor drive.
Each of the prior art arrangements, however, has shortcomings and disadvantages in terms of efficiency of operation and distortion across the entire range of operating speeds of an electric motor.